Cold cathode carbon film

ABSTRACT

A carbon film having an area of insulating material surrounded by an area of conducting material, and an area of material between the area of insulating material and the area of conducting material having a graded dielectric constant which varies from high to low from the area of insulating material to the area of conducting material.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of U.S. ProvisionalPatent Application Ser. No. 60/062,934 filed Oct. 21, 1997.

TECHNICAL FIELD

The present invention relates in general to field emission devices, andin particular, to a cold cathode carbon film for use as a field emitter.

BACKGROUND INFORMATION

Cold cathodes are materials or structures that emit electrons with theapplication of electric fields without heating the emitter significantlyabove room temperature. Examples of cold cathodes are small metal tipswith sharp points that are fabricated together with a grid structurearound the tips such that an appropriate bias placed between the gridstructure and the tips will extract electrons from the tips whenoperated in a suitable vacuum environment (Spindt emitters).

Diamond, diamond-like carbon (DLC) and other forms of carbon films havealso been investigated for use as cold cathode electron emitters formany applications, such as flat panel displays, microwave deviceapplications, backlights for liquid crystal displays (LCDs), etc. Manydifferent techniques for growing the carbon films were tried resultingin a wide variety of carbon films. The mechanism for electron emissionfrom these carbon films is not clear and is the subject of muchinvestigation. What has been found consistently is that electrons arenot emitted uniformly from the carbon cold cathodes, but are insteademitted from specific areas or sites of the carbon film. These areas arethe emission sites (ES). The density of these sites in a unit area isreferred to as the emission site density (ESD).

Researchers recognized early on that the negative electron affinity ofthe hydrogen terminated <111> and <100> faces of diamond may beimportant. A material having negative electron affinity (NEA) means thatif an electron is in the conduction bands of the material, this electronhas no barrier to prevent it from leaving the material if the electrondiffuses to the surface having the NEA property.

The question for diamond has always been how to get an electron into theconduction band of diamond. This is not an easy question since diamondis an insulator with a very wide energy gap (5.5 eV) between theconduction band and the valence band. For an insulator at roomtemperature with this large a band gap, the population of electrons inthe conduction band is too small to support any substantial emissioncurrent. Researchers have speculated that the electrons are injectedinto the diamond from a back side contact.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates an apparatus for measuring the dielectric constant ofa material;

FIG. 2 illustrates results of one area of a carbon film showing thedielectric properties and the field emission properties;

FIG. 3 illustrates a field emitter device configured in accordance withthe present invention; and

FIG. 4 illustrates a data processing system configured in accordancewith the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forthsuch as specific word or byte lengths, etc. to provide a thoroughunderstanding of the present invention. However, it will be obvious tothose skilled in the art that the present invention may be practicedwithout such specific details. In other instances, well-known circuitshave been shown in block diagram form in order not to obscure thepresent invention in unnecessary detail. For the most part, detailsconcerning timing considerations and the like have been omitted inasmuchas such details are not necessary to obtain a complete understanding ofthe present invention and are within the skills of persons of ordinaryskill in the relevant art.

Refer now to the drawings wherein depicted elements are not necessarilyshown to scale and wherein like or similar elements are designated bythe same reference numeral through the several views.

The inventor produced films for testing to characterize emission sitesat better than 100 nm spatial resolution using a modified atomic forcemicroscope.

Referring to FIG. 1, a modified scanning microscope was operated in twomodes. In the first mode the tip 104 was placed touching the surface ofthe sample 101 (mounted on sample holder 102) and scanned across thesurface 101 to measure the physical topography. The height of the tip104 was detected by bouncing a light beam from light source 105 off ofthe end of the tip 104 and reflected into a position detector 106. Theposition of the light hitting the detector 106 is dependent on theheight of the needle tip 104. In another mode, the tip 104 was placedabout 100 nm away from the surface as shown in FIG. 1. A voltage biasfrom source 103 was placed on the needle tip 104 while the tip 104 wasscanned across the surface. By biasing the tip 104, an electric chargewas placed on the tip 104 relative to the surface. The material reactedto the charge on the tip 104 by placing charges on the surface in such away as to form what appears to be an image charge inside the material101. The strength of the image charge is dependent on the dielectricconstant (∈) of the material 101 as given by the equation:

−q′=q*(∈−1)/(∈+1)

Here q′ is the magnitude of the image charge and q is the charge placedon the tip 104. Since the image charge is of opposite polarity to thecharge on the tip 104, an attractive force develops between the needletip 104 and the surface of the substrate 101. This force deflects theneedle 104. The magnitude of the force is detected by the position ofthe light hitting the detector 106. By scanning the needle tip 104across the surface, a mapping of the relative dielectric constant acrossthe surface is obtained. Simultaneously, if the bias on the tip 104 ishigh enough, electrons from the carbon film 101 can be field emittedfrom the surface of the sample 101 to the tip 104. By monitoring thecurrent to the tip 104, the emission sites of the carbon film 101 can belocated. Thus this instrument can map simultaneously the spatialemission properties of the sample 101 and the dielectric properties ofthe material 101, allowing us to correlate the results.

FIG. 2 shows the results of one area of the carbon film 101 showing boththe dielectric properties (left side image) and the field emissionproperties (right side image). What was discovered was that the fieldemission sites are correlated with specific dielectric properties of thesample 101. The features that are correlated to the emission sites arecharacterized by a dark area surrounded by a ring of bright area. Theylook like small volcanoes. Examples of these are features labeled A, B,and C. The emission sites are actually centered on the dark part of thevolcano features. This corresponds to an area of material having arelatively low dielectric constant surrounded by a ring of material 101having a relatively higher dielectric constant. Classically, thedielectric constant is related to the conductivity of the material. Wecorrelate the areas of relatively high dielectric constant to materialthat is more conductive. We correlate the areas of relatively lowdielectric strength to material that is more insulating. Since the filmwas grown by a diamond CVD process, we can conclude that the insulatingmaterial is diamond and that the conductive ring is amorphous orgraphitic carbon.

We also note that the dielectric distribution going towards the centerof these volcanoes is not abrupt but instead is gradual until it reachesthe dark center of the volcano. This suggests that the dielectricconstant of the material 101 varies gradually towards the center of thevolcano feature. In other words, the material surrounding the diamondhas a graded dielectric constant, the interfaces are not abrupt, butgradual. One of the emission sites (site A) has the volcano feature aswell as a well defined area of high dielectric constant next to it.

We also note that there are other features in the dielectric map that donot correspond to emission sites. Two features marked E and F do nothave the dark centers of the volcano features. Another volcano-likefeature (labeled G) is not correlated with an emission site. Note thatthis feature also is not surrounded by a significant conducting ring asthe other features A, B, C. Feature H is a large patch of low dielectricmaterial that also does not have a ring structure around it. It also isnot an emission site.

Finally we note that the intensity of emission from different sites isnot uniform. The site marked D has the smallest emission intensity ofthe sites that emit. Its volcano features are hardly discernible.

Thus we find that a certain structure promotes electron field emissionfrom the diamond films. These structures consist of a small diamondparticle (less than 2000 Å in diameter) surrounded by a material thathas a dielectric constant that changes gradually in a volcano-likestructure. It is believed this structure is necessary to promoteinjection of electrons into the low dielectric material which ispresumably diamond. Once in the diamond conduction band, these electronshave little or no barrier for emission because of the low or negativeelectron affinity of the diamond surfaces.

Referring next to FIG. 3, there is illustrated field emitter device 80configured with a film produced in accordance with the inventiondiscovered above. Device 80 could be utilized as a pixel within adisplay device, such as within display 938 described below with respectto FIG. 4.

Device 80 also includes anode 84, which may comprise any well-knownstructure. Illustrated is anode 84 having a substrate 805, with aconductive strip 806 deposited thereon. Then, phosphor layer 807 isplaced upon conductive film 806. An electrical potential V+ is appliedbetween anode 84 and cathode 82 as shown to produce an electric field,which will cause electrons to emit from film 501 towards phosphor layer807, which will result in the production of photons through glasssubstrate 805. Note that an alternative embodiment might include aconductive layer deposited between film 501 and substrate 101. A furtheralternative embodiment may include one or more gate electrodes (notshown).

As noted above, field emitter device 80 may be utilized within fieldemission display 938 illustrated in FIG. 4. A representative hardwareenvironment for practicing the present invention is depicted in FIG. 4,which illustrates a typical hardware configuration of workstation 913 inaccordance with the subject invention having central processing unit(CPU) 910, such as a conventional microprocessor, and a number of otherunits interconnected via system bus 912. Workstation 913 includes randomaccess memory (RAM) 914, read only memory (ROM) 916, and input/output(I/O) adapter 918 for connecting peripheral devices such as disk units920 and tape drives 940 to bus 912, user interface adapter 922 forconnecting keyboard 924, mouse 926, speaker 928, microphone 932, and/orother user interface devices such as a touch screen device (not shown)to bus 912, communication adapter 934 for connecting workstation 913 toa data processing network, and display adapter 936 for connecting bus912 to display device 938. CPU 910 may include other circuitry not shownherein, which will include circuitry commonly found within amicroprocessor, e.g., execution unit, bus interface unit, arithmeticlogic unit, etc. CPU 910 may also reside on a single integrated circuit.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A method of operating a field emission device,comprising the steps of: providing a carbon film deposited on asubstrate; applying an electric field to the carbon film to therebycause an emission of electrons from a diamond material, wherein theelectric field causes an injection of electrons from a graphitic carbonmaterial through a material surrounding the diamond material having agraded dielectric constant from high to low to the diamond material. 2.A carbon film comprising: an area of insulating material surrounded byan area of conducting material; and an area of material between the areaof insulating material and the area of conducting material having agraded dielectric constant which varies from high to low from the areaof insulating material to the area of conducting material.
 3. The carbonfilm as recited in claim 1, wherein the insulating material is diamondand the conducting material is amorphous or graphitic carbon.